There is a need for a reliable button that does not use or need moving parts or mechanical contacts, to be used in, for example, elevator applications. It is desirable that such a button be aesthetically pleasing in appearance and use, highly reliable, low in cost and not be activated by extreme environmental changes.
It is known that the human body has some amount of capacitance to ground. A basic, previously known concept or approach is to monitor a button face for capacitance to ground, and, if a certain amount (or greater) capacitance is present, to activate the button.
In the present invention, this monitoring of the button surface is accomplished by monitoring the phase shift of a signal applied to the face of the button.
However, due to the residual impedance of the button and the presence of external influences, the steady state phase shift of the button can fluctuate. Such external influences include, for example, temperature changes, cleaner residue build-up and other deposits or extreme environmental changes, etc. This fluctuation has the potential to falsely activate a button, which is based on the monitoring of phase shift, although in fact no human operator is touching the button.
A distinguishing characteristic of such exemplary fluctuation is that it would occur slowly over a period of time relative to a phase shift induced by the presence of a person. To avoid this problem, in a first "auto-balancing" embodiment of the invention the steady state phase shift fluctuation is compensated for by slowly auto balancing to all phase shifts The rate of auto balance is set to be faster than the rate of change of the steady state phase shift. The rate of change of phase shifts induced by a person is faster than the auto balance rate, which allows the button to be operated. In a second, "constant pressure" embodiment, another way to compensate for the exemplary fluctuations is to simply set a threshold for activation higher than any normal fluctuations.
A second potential source of false activations of such a button are phase shifts induced by transient noise. These potential transients include, for example, electromagnetic interference (EMI) and radio frequency interference (RFI).
A distinguishing characteristic used in the invention to help prevent false activations caused by transient noise is that such noise would not last for a long period of time relative to the interaction time with a person. Activations due to transient noise thus are avoided in the invention by delaying the activation of the button for a certain minimum period of time.